In a construction machine including a hydraulic actuator such as a hydraulic cylinder (hereinafter, occasionally simply referred to as a cylinder), an oil stored in a hydraulic oil tank is fed to the hydraulic actuator by a hydraulic pump. After the hydraulic actuator is operated, the oil is returned to the hydraulic oil tank. The hydraulic oil tank includes an oil filter. The oil returned from the hydraulic actuator is filtrated with the oil filter and stored in the hydraulic oil tank (see, for instance, Patent Literature 1).
The oil filter includes a filter case and a filter element housed in the filter case. The oil fed through an intake port of the filter case passes through a filtering medium of the filter element to remove foreign substance contained in the oil before the oil is discharged from a discharge port.
Each time foreign substances are removed by the filter element used in the oil filter, the filtering medium becomes clogged with the foreign substances, so that the oil gradually becomes difficult to pass through the filter element. In such a case, there has been known a technique of providing a bypass channel through which an upstream side of the filter element communicates with a downstream side of the filter element, providing a valve in the bypass channel, and, when a differential pressure between the upstream side and the downstream side of the filter element reaches a predetermined value or more, notifying an operator of the clogging of the filter element while opening the valve in the bypass channel to keep the flow of the hydraulic oil (see, for instance, Patent Literature 2 and Patent Literature 3).